Several materials are known in the art which are useful for providing electron emission in vacuum devices such as field emission devices. These prior art field emissive materials include metals such as molybdenum or semiconductors such as silicon. Because these materials require high electric fields, on the order of hundreds or thousands of volts per micrometer, emitter structures which include the emissive material are typically shaped to include, for example, sharp tips which function to locally increase the electric field and provide field strengths adequate to yield electron emission. Reasonable emission characteristics with stability and reproducibility necessary for practical applications have been demonstrated. However, the control voltage required for emission from these materials is relatively high (around 100 V). High voltage operation is undesirable for several reasons. For example, contaminant charged species discharged at the electron receiving material are accelerated to high velocities, thereby exacerbating damage caused by the bombardment by these species on elements of the device. Also, higher voltages require greater power consumption for a given current density. Another undesirable consequence of the use of these materials is that the fabrication of uniform sharp tips is difficult, tedious and expensive.
Also known in the art is the use of certain carbon-based materials to form electron emissive films. These include diamond, diamond-like carbon, and polycrystalline diamond films. These materials provide emission at low voltages so that the formation of field-enhancing, sharp-tipped geometries are therefore not required. This substantially simplifies the fabrication process of the emissive structure and substantially reduces costs. However, prior art carbon films have been plagued with unacceptably low emission site densities, on the order of tens or hundreds of sites per square centimeter. The low emission site density results in poor uniformity. Diamond does not emit electrons in a stable fashion due to its insulating nature; that is, the electrons which are emitted are not easily replaced due to the difficulty of providing a current path through the diamond. This results in a low density of emission sites in the film, resulting in a film which is unsuitable for use in applications such as field emission displays. Prior art methods for improving the electron emitting properties of diamond include doping the diamond to provide n-type or p-type semiconducting diamond. However, the n-type doping process has not been reliably achieved for diamond, and the p-type semiconducting diamond is not useful for low-voltage emission since it requires voltages in excess of 70 volts per micrometer to generate emission current densities on the order of 0.1 mA/mm.sup.2. Other schemes for improving the conductivity of diamond films involve the formation of a high density of crystallographic defects within the film.
Electron emissive films have been formed by plasma enhanced chemical vapor deposition (PECVD). Amorphous carbon hydrogen-containing films (a-C:H) deposited by PECVD have been developed for field emission. These films have a low turn-on field within a range of about 15-30 V/micron. However, the emission site density of these films is low, on the order of tens of sites per square centimeter. Another undesirable characteristic of hydrogenated carbon films is that they exhibit poor temporal stability due to hydrogen out-diffusion.
The cathodic arc vapor deposition technique is primarily employed in the formation of coatings or films for use in tribological applications, such as the formation of wear-resistant coatings for cutting tools, bearings, gears, and the like. These wear-resistant coatings have been made from plasmas formed from titanium or graphite sources. When a titanium source material is used, a reactive gas such as nitrogen is often introduced into the deposition chamber during the vaporization of the titanium source. The nitrogen gas reacts with the titanium, and the coating plasma within the chamber comprises Ti, N.sub.2 and TiN. The TiN forms a coating that has been found to be a very durable coating. A graphite source material is used to form diamond-like carbon (DLC) films, tetrahedral amorphous carbon (ta-C), and carbon nitrogen (C:N) films.
Prior art carbon films produced by cathodic arc vapor deposition techniques are used for tribological applications, such as the formation of extremely hard and resistive thin films. The carbon films are deposited on substrates typically made of metal and impart wear resistance to the metallic surface. The cathodic arc vapor deposition process has been modified to enhance and optimize the tribological properties of the films. In one scheme, a hydrogen free ta-C film is deposited by the evaporation of a graphite source by a filtered vacuum arc; the resulting microstructure is amorphous, having various ratios of sp.sup.3 /sp.sup.2 hybridization of the carbon and forming unsegregated phases.
Also known in the art are a wide range of heat-treated carbons which are prepared by heating an organic precursor to temperatures in excess of about 1000.degree. C. At these high temperatures some carbon precursors are known to graphitize. Within the temperatures of about 500-900.degree. C., organic precursors tend to form chars; at about 1000.degree. C. nitrogen and oxygen are evolved; and at about 1200.degree. C. hydrogen is evolved. At sufficiently high temperatures, some carbons will begin to graphitize. Such high temperatures, however, exceed the temperature tolerances of glass, a material typically employed in the substrates of field emission displays.
There exists a need for an electron emissive film which emits electrons at low electric field strengths, is suitable for use in field emission devices, provides uniform electron emission and a high density of emission sites, and has a turn-on field of less than 30 V/.mu.m. Additionally, there exists a need for a method for forming such a film which includes deposition temperatures that are compatible with the temperature constraints of glass substrates used in field emission devices.